LinuxKPI: Add linux/iosys-map.h header

Sponsored by: Serenity Cyber Security, LLC
Obtained from: OpenBSD
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42799

(cherry picked from commit f8ab2f5bae1d4e4e13f95f4738c9bb49fcda6261)